Think you should definitely go with an I name if you want to keep it a vowel. I'm personally not a fan of repeating first letters for siblings. I think Ivy is your best best with sibling names. I like Isla, but don't think it sounds the best with Ella. IvyLillian gets my vote!

I still really like Zoe with your girls' names. I know it doesn't start with a vowel, but man it just makes so much sense with those other names. Really Zoe sounds great with all of those middle names, but I think I like ZoeJean the best. Its so great!

Some names I would avoid:Eva/Eve - Too similar to EllaIsabella - It literally has Ella in itSophia - The ending is too similar to the ending of OliviaElena - Sounds similar to Ella
Try to avoid nicknames that could sound like your daughter's nicknames in general.